Step 1: Think about the direction of effort.
Social loafing is a fall in effort in groups. So the right option must give a reason why effort goes down. Any option that gives a reason for effort going up cannot be the answer.
Step 2: Test option (2).
Here motivation goes up. That is the opposite of loafing, and individual evaluation is actually the usual cure for it. So (2) is out.
Step 3: Test options (3) and (4).
Good coordination and a strong sense of belonging are positive group features. They help group work and do not lower effort. So (3) and (4) are out.
Step 4: Test option (1).
If the output is pooled, each person's share of blame or credit is small. Feeling less responsible leads to less effort. This is the diffusion of responsibility reason, so (1) is right.
Step 5: Put it in plain words.
Imagine a team pulling a rope or a group project with one shared mark. A member can think that others will cover for him or her, so the personal push becomes smaller. That smaller push, caused by lower felt responsibility, is exactly what we call social loafing.
Final Answer:
Reduced personal responsibility explains social loafing, so option (1) is correct.\[ \boxed{1} \]